The present invention generally relates to methods and systems for controlling multiple temperature modification sources that service a building or other enclosed space, and more specifically, to methods and systems for controlling which temperature modification source(s) is used at any given time.
Buildings and other structures often include multiple temperature modification devices or systems to help control the temperature and/or other environmental parameters within the building. For example, some buildings include both a fossil fuel furnace and a heat pump. In this example, and under some conditions, it is more cost efficient to use the heat pump to heat the building space than the fossil fuel furnace. However, under other conditions, such as when the outside air temperature drops and the efficiency of the heat pump drops, it may be more desirable to activate the fossil fuel furnace and de-activate the heat pump. In this example, the fossil fuel furnace can be considered an auxiliary heat source.
In many cases, and continuing with the above example, it is undesirable to operate both the heat pump and fossil fuel furnace at the same time. That is, either the heat pump is enabled or the fossil fuel furnace is enabled, but not both at the same time. In many cases, the determination of whether the heat pump or the fossil fuel furnace provides heat to the building is based only on the outdoor air temperature. That is, when the outdoor air temperature drops below a heat pump lockout temperature, the control system locks out the heat pump and activates the fossil fuel furnace. The fossil fuel furnace is then used until the outside air temperature rises above the heat pump lockout temperature.
It has been found that using only the outside air temperature to determine when the system transitions between the heat pump and the fossil fuel furnace can result in inefficient operation. For example, under some conditions the heat pump may be able to satisfy the heat load of the building even when the outside air temperature is at or below the heat pump lockout temperature. For example, on a sunny day the sun may provide sufficient green house heating to the building to reduce the heating load so that the heat pump can satisfy the heat load, even though the outside air temperature is at or near the heat pump lockout temperature. In addition, under some conditions the heat pump may not be able to satisfy the heat load of the building even though the outside air temperature is above the heat pump lockout temperature. For example, on a windy night the heat load of the building may exceed the heating capacity of the heat pump even though the outside air temperature is above the heat pump lockout temperature. It has been found that there are many factors that may effect when it is most appropriate to transition between a primary heating/cooling source and an auxiliary heating/cooling source including, for example, building construction and insulation, current weather conditions (sun, clouds, wind, humidity, precipitation, time of day, season, etc.), inside air temperature set point, outside air temperature, as well as other factors.